Federal and state health officials are trying to identify any more cases of a deadly fungal meningitis that they suspect resulted from an injected steroid commonly used to treat back pain. The drug, made by a Massachusetts company that has suspended operations, was apparently contaminated by a common leaf mold. So far 35 people have gotten sick in six states and five have died. But thousands more may have received the tainted drug. The incubation period can be as long as a month, so the danger period may not be over until late October.

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Federal health officials say 35 people in six states have gotten a rare and dangerous form of meningitis. It came from a contaminated drug that was injected into their spine to treat back pain. Five patients have died so far. And officials say patients who may have had spinal injections for back pain as long ago as July may be at risk for the unusual infection. NPR's Richard Knox has more details.

RICHARD KNOX, BYLINE: These meningitis cases are due to a fungus that rarely causes the brain infection in humans. Somehow it got into a drug called methylprednisolone, a steroid often used to ease severe back pain. The Food and Drug Administration says it has no idea how many doses of the contaminated drug may have been shipped out since July. About 75 clinics and hospitals in 23 states got the tainted drug. It came from a Massachusetts company called the New England Compounding Center which shut down voluntarily this week. The FDA has recalled three lots of the drug. Dr. Brad Park of the Centers for Disease Control and Prevention says the outbreak is not over.

DR. BRAD PARK: Unfortunately, despite the current recall, we expect to see additional cases as this investigation unfolds.

KNOX: More common types of meningitis can have sudden and dramatic symptoms. But Park says this form may be subtle at the beginning. It can cause headaches, nausea, stiff neck and stroke-like symptoms such as difficulty finding words, numbness or weakness.

PARK: It is possible that if patients are identified soon, and started on appropriate anti-fungal therapy, some of the unfortunate consequences may be averted.

KNOX: The FDA has found evidence of fungal contamination in a vial of the steroid at the Massachusetts company. Ilisa Bernstein of the FDA says medical facilities should pull all of the company's products off their shelves.

ILISA BERNSTEIN: Investigation into the source of the outbreak is ongoing. But, given the severity of the illnesses we have seen so far, we believe these precautionary measures are warranted to protect public health.

KNOX: The first case was identified last month at Vanderbilt University Medical Center in Nashville. Dr. William Schaffner is an infectious disease expert there.

DR. WILLIAM SCHAFFNER: Dr. April Pettit, one of our infectious disease colleagues, was caring for a patient with a troublesome form of meningitis.

KNOX: The man, in his 50s, wasn't responding to conventional treatment. So Dr. Pettit sent a sample of his spinal fluid to the lab.

SCHAFFNER: When the microbiology laboratory informed her that aspergillis had been recovered from culture, this was a stunning and totally unexpected result.

KNOX: Aspergillis is the name of a common leaf mold. The specialist learned the man had gotten a spinal injection a couple of weeks earlier. She put two and two together and called state health authorities. They've identified 18 Tennessee patients infected by the contaminated medicine. Three have died, including Dr. Pettit's patient. Schaffner says fungal meningitis requires weeks and months of treatment. Survivors may end up with permanent neurologic damage. Officials say this kind of meningitis, unlike more common kinds, can't be passed from person to person. Richard Knox, NPR News.

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